Cardiac sarcoplasmic reticulum (CSR) plays a central role in the regulation of cardiac contractility and in relaxation of the heart. It is known that short periods of ischemia result in damage to the CSR, but the character and cause of the damage is unknown. The character of this damage can be assessed only by comparison of damaged CSR to normal CSR. However, our understanding of normal CSR function, particularly the mechanisms of calcium efflux, is only rudimentary. The aim of the proposed research is to examine the mechanism of calcium efflux through several possible routes, but with greatest emphasis on the putative calcium channel. Several CSR preparations will be used. Crude CSR vesicles will be prepared according to established procedures. These vesicles will be further fractionated into two classes: one typified by high Ca-ATPase and oxalate-supported calcium uptake rates but no sensitivity to ryanodine and another possessing high Ca-ATPase, little calcium uptake but marked stimulation of uptake by ryanodine. These ryanodine-sensitive vesicles are believed to contain the putative calcium channel. Identification of the channel will be performed by polyacrylamide gel electrophoresis under denaturing conditions following photoaffinity labeling with 3H-ryanodine. Calcium uptake and release by the ryanodine-sensitive vesicles will be monitored using 45Ca and Millipore filtration and spectrophotometry using arsenazo III. The regulation of efflux through the calcium channel will be investigated using rapid-scanning spectrophotometry. Fulfillment of the aims of this proposal will significantly enhance our understanding of normal CSR function and thereby allow a better understanding of its dysfunction in disease.